I’ve come across this fact stating something along the lines of ‘a 40% alcohol solution is ineffective at killing germs, but anything between 60% – 90% is.” Why is that?
The fact I’m referring to can be found [here](https://www.insider.com/does-alcohol-kill-germs), [here](https://www.cdc.gov/infectioncontrol/guidelines/disinfection/disinfection-methods/chemical.html#Alcohol) and [here](https://blog.gotopac.com/2017/05/15/why-is-70-isopropyl-alcohol-ipa-a-better-disinfectant-than-99-isopropanol-and-what-is-ipa-used-for/)
In: Biology
I always thought it was the rapid evaporation/drying that actually killed the germs but reading other comments it seems the alcohol dissolves the germ.
From what I understand, if the solution is **too dilute (under 60% or whatever)**, it doesn’t have enough chemical “ammo” to *quickly* “burn” through cellular membranes and denature the proteins inside. It works against some germs, but not the more resilient ones.
If the solution is **too concentrated (90% and higher)**, it actually has *too much* chemical ammo; instead of dissolving/permeating through the membrane, it coagulates the outer cellular membrane, which in turn creates a sort of protective shell that slows the rate at which the alcohol can attack the cell. Additionally, these higher concentrations evaporate more quickly, thereby reducing contact time.
That **sweet spot of 70%** is just beefy enough to tear through the cellular membrane and fuck shit up–but *isn’t beefy enough* to chemically sear the membrane into a protective shell. Since water evaporates more slowly than alcohol, having a higher percent of water means it will evaporate more slowly, thereby increasing contact time. IIRC, this is the concentration that is well suited for killing spores.
Lastly, it’s worth noting that isopropyl alcohol and ethyl alcohol are different alcohols, and each one behaves better (or worse) against certain germs. Here’s the CDC’s page on [chemical disinfectants](https://www.cdc.gov/infectioncontrol/guidelines/disinfection/disinfection-methods/chemical.html).
40% is too diluted to penetrate, anything over 90% evaporates before it can fully penetrate. 60 to 90 is just right.
Alcohol [needs TIME to kill](https://www.researchgate.net/profile/Aidan_Nikiforuk/publication/311322233/figure/fig1/AS:434882271813636@1480695421701/Standard-Ebola-Kill-Curves-Reduction-in-Ebola-virus-titres-when-the-a-Mayinga-b_Q320.jpg), it’s not instantaneous. All chemicals need time, actually. A lot of the easy germs die within seconds, but some of the more resistant ones can survive for some minutes. For 70% rubbing alcohol, 10 minutes is the recommended time, as most of the germs will be dead by then for that concentration.
So what happens is, if you less alcohol (and more water) in the mixture, 60% 50% etc., then you need more than 10 minutes for the same kill power.
And the problem is that this stuff evaporates; try to pour some rubbing alcohol on your hands, it will evaporate probably well before 10 minutes, your hands will be dried up by the 2 minute mark and the killing will stop at 2 minutes instead of 10, leaving some germs alive.
If you have more concentration of alcohol, 90%, it kills a bit faster, say 8 minutes, but alcohol evaporates A LOT faster than water, so your hands will be dry within 30 seconds, which is basically worse.
Sweet spot between “hands wet the longest time” and “killing power” is 70%.
Latest Answers